Q. I received SQL syntax errors of MySQL or SQLite3 for union statement like rel1 `unionAll` rel2 `unionAll` rel3.

A. The only sub-set SQL syntax support of MySQL or SQLite3 causes this problem.
You can avoid this problem like relation (query $ rel1 `unionAll` rel2) `unionAll` rel3.
There is more detail information at the discussion on this pull-request.